This invention relates generally to portable hand-held power saws. More particularly, this invention relates to safety systems for preventing chainsaw injury to humans and for limiting the depth of cut of a chainsaw into a target object such as a roof or a tree.
In the field of firefighting and fire suppression in burning structures, rotary saws and chainsaws are used to cut ventilation openings. Carbide-tipped chainsaws are required for firefighting because of the wide variety of materials encountered when cutting through building structure surfaces. Ventilation openings are intentionally made by firefighters in a burning structure to reduce or remove concentrations of heat, smoke, and fire gases from a burning structure, or to channel and redirect the travel of fire.
FIG. 1 shows a typical approach for cutting a ventilation opening 20 through a roof 22 of a burning structure; only a portion of the roof of the burning structure is shown. A firefighter 24, wearing a fire-protective suit 26 including boots 28 and gloves 30, ascends to the roof. The firefighter often wears a self-contained breathing apparatus 32 including a tank 34 connected to deliver breathable air to the firefighter, for example by way of a protective face mask and hardshell helmet 36 having a face shield. The firefighter carries a chainsaw 38 for cutting the ventilation opening.
In actual practice two firefighters (only one is shown) work as a team when cutting these holes; one is the cutter (in this case, firefighter 24) and the other is the safety guide (not shown). The guide stands behind the cutter at arms length for guiding the cutter to avoid injury to the cutter and to others working close to the cutter. However, the guide firefighter is omitted from the drawing and from this discussion to simplify this explanation.
Firefighter 24, after selecting the location for cutting ventilation hole 20, starts chainsaw 38, typically driven by a two-cycle gasoline engine powerhead. To initially diagnose the roof structure and composition, the rafters and the nature of the fire, the firefighter typically makes a 45-degree inspection cut along a diagonal line 54. Building construction most often is parallel to or at 90-degrees to any exterior wall; therefore, a 45-degree cut will ensure that the chainsaw intersects a rafter 52.
A covering 48 and a base 50 of roof 22 are penetrated by this inspection cut, but underlying supporting rafters 52 are not cut. Instead, the firefighter rolls the rafter with the chainsaw--that is, the firefighter, upon feeling the rafter, lifts the saw over the rafter without cutting through it--and continues cutting for about another linear foot. The firefighter makes two additional cuts to complete a right-triangular inspection opening 46. If satisfied with the information obtained from inspection opening 46, the firefighter enlarges it along a perimeter cut 47 to make ventilation opening 20. Usually four perimeter cuts 47 are made, to define a generally rectangular opening 20. Before ventilation hole 20 is fully formed, additional cuts 56 are usually made in roof 22 parallel to rafters 52 to make the cut roof sections easier to remove with other tools (not shown).
Advantages are offered by chainsaws that significantly simplify many ventilation operations. Chainsaws are effective on roofs having a base 50 made of wood or metal. Also, chainsaws will cut heavy layers of roof composition common on older types of roof construction, and built-up layers of insulation and composition on metal deck roofs. Additionally, chain saws have excellent reach and balance which is beneficial to the firefighter-operator during cutting operations. Unlike other power tools such as rotary saws, chainsaws can be used to "feel" or "read" the construction--that is, rafters or joists--to determine their location and spacing for effectively placing cuts in roof decking.
However, chainsaws also exhibit significant safety hazards that must be considered when operating the saw. Most of the chain and cutter teeth are continuously exposed, requiring constant operational and safety attention by the chainsaw operator. Although most chain saws are equipped with a centrifugal clutch, the chain can continue to spin at idle; therefore, extreme caution by the operator is required when turning with a running chainsaw.
To avoid being injured by flying objects, the operator when cutting must strive to keep his body out of alignment with respect to the chainsaw; that is, the chainsaw operator needs to hold the chainsaw diagonally, and not perpendicularly, with respect to his body. Chainsaws often dislodge and throw objects such as nails, small rocks and splinters for a considerable distance with significant force. Eye protection must be worn by all personnel involved in cutting operations with a chainsaw. Proper positioning of the chainsaw with respect to the firefighter's body can prevent injury from a broken chain, flying debris, or kickback of the saw.
Kickback is particularly dangerous because it is so violent and it occurs so fast that the chainsaw operator cannot react quickly enough to avoid serious injury. The two common types of kickback are rotational kickback and linear kickback. Rotational kickback is the reaction which occurs when (see FIG. 1) the chainsaw cutter chain 62, at an upper chain tip portion 64 moving along the last couple of inches of the upper section of the chainsaw nose 66, suddenly stops against an object being cut. The chainsaw reacts by kicking back violently in an upwardly rotating arc, and thus back toward the head, shoulders and chest of the operator, potentially causing serious injury.
Linear kickback is a vigorous push reaction which occurs when the cutter chain is buried in a cut and then the object being cut closes around the chain, pinching it against the chainsaw underlying supporting structure so the chain stops spinning. The chainsaw reacts by kick-pushing the operator backward, possibly causing the operator to lose control of the chainsaw or lose balance to the point of falling off the roof. One technique for lessening the chance of kickback occurring is to run the saw at full throttle; however, this approach entails its own risk because, while kickback may happen less often when running at full throttle, when kickback does occur it will be even that much more violent.
Existing chainsaws are not cut-depth limited. That is, chainsaws will cut to a depth limited only by the length of the elongated chainsaw structure, shown in FIG. 1 as a guide bar 70, that supports the traveling cutter chain during chainsaw operation. The guide bar is available in lengths ranging from twelve to thirty-six inches. The most commonly used lengths measure sixteen, twenty and twenty-four inches, so the corresponding depths of cut can as a result be sixteen, twenty and twenty-four inches. To limit the depth of cut, firefighters must continuously probe and "feel" their way along, while making the cut, to determine the proper depth of cut for each cutting operation. The best "feel" is achieved when the chainsaw is held verticle, but this is an impractical and unnatural orientation. The more practical and natural way to cut is to hold chainsaws at about a forty-five degree angle with respect to the surface being cut, a surface that is usually horizontally oriented.
Therefore firefighters must "feel" their way as they cut to minimize the chance of cutting through building parts that should not be cut, such as rafters and other structural members, electrical wires and conduits, and plumbing pipes. Firefighters are forced to move slowly in a hazardous environment at a time when speed lessens their risk; firefighters must move deliberately to cut at a measured pace to feel their way along an opening they are cutting in the roof of a burning building.
A firefighter's ability to move quickly when using a chainsaw becomes more important if one considers the other factors making their work inherently hazardous. For example, roughly 70% of fires are fought between the hours of 2:00 to 5:00 A.M.; thus, a firefighter working on the roof of a burning building works with a diminished ability to see because he is literally working in the dark. Firefighters are often awakened out of their sleep and then within minutes can be fighting a fire while not yet fully awake and alert.
Contruction techniques create firefighter hazards. Modern construction methods begun roughly around 1960 produce buildings, particularly residential housing, that collapse in a fire much more quickly than buildings built before 1960. Thus firefighters must move fast for their own survival. Pre-1960 buildings structurally fail after burning for about twenty minutes. In contrast, post-1960 buildings often fail in only three to nine minutes. The on-site commander of the firefighters considers the probable type of construction method used when deciding whether or not to send firefighters onto the roof.
Construction methods before 1960 erected lumber pieces on-site and nailed these pieces together to join them; the finished structures were and are rugged. Since 1960, modern light-weight construction methods assemble lumber pieces off-site into assemblies that are then hauled to and erected at the jobsite. To form pre-built joist rafter assemblies, a an off-site assembler hydraulically squeezes metal gusset plates having one-eighth inch spikes into the lumber. On-site the builder combines these triangles-within-triangles rafter assemblies into a roof structural support system. In contrast to pre-1960 buildings, the resulting post-1960 building are less rugged and more susceptible to faster structural failure during a fire.
A fire quickly heats these metal gusset plates, which transfer their heat to burn the wood they join. Gusset plates quickly become localized hot spots that intensify the heating rate of the underlying wood. As a result the wood burns more quickly. Further, metal gusset plates expand faster than wood when heated, moving the spikes relative to the wood; this movement loosens structural joints to cause roof structural failure sooner. Studies show that failure of one gusset plate joint triangle causes at least three more to fail; like dominoes the entire roof structure falls soon thereafter, taking with it any firefighters above and below.
Firefighters wear bulky protective gear and other equipment weighing anywhere from 40 to 80 pounds; this diminishes the ability of a firefighter's body to feel how a chainsaw is responding to the material it is cutting. Further, this weight and bulkiness makes it difficult to respond to such hazards as chainsaw kickback. Fires spread quickly through burning buildings; firefighters need to cut a ventilation hole quickly and then leave.
Consequently, a need exists for chainsaw systems that are safer to operate and therefore that reduce or eliminate the hazards arising from normal chainsaw operation. In particular, firefighters need a safer chainsaw which enables them to work quickly with a chainsaw to make cuts of limited depth in a burning environment that by its nature is dangerous.